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July 2016 • The PCB Magazine 27
isolate the interactions of the waves with the
material under test from any parasitics intro-
duced by the test fixture. Another significant
challenge at these high frequencies is that cur-
rent is concentrated at the "skin" of metal sur-
faces. As frequencies increase, the microstruc-
ture of metal surfaces contributes more signifi-
cantly to overall loss or degradation, and makes
it nearly impossible to isolate the impact of the
dielectric losses separate from the metal.
Introduction
In an effort to potentially determine stan-
dardized test methods at these frequencies,
seven members of IPC D-24C Task Group devel-
oped a round-robin to measure ɛ
r
and tan δ for
various PCB materials using different methods
of their choosing and compare results.
First, this paper details the problem followed
by a description of the various evaluation meth-
ods being considered; each method is described
with sufficient information to allow for third
party replication. Next, the results from each
labs independent dielectric property character-
izations are presented and subsequently com-
pared. Finally, this paper will discuss each meth-
ods pros and cons and any conclusions or next
steps.
Each test lab participant measured ten cir-
cuit board material samples up to the highest
frequency for which they could provide valid
data. Each participating test lab measured mate-
rial from the same lot. The circuit board materi-
als for testing were constrained to the following
general properties:
• 0.5 oz Copper Clad (18 μm thick)
• Dielectric Thickness: 100-150 μm
• Relative Permittivity (ɛ
r
): 2.0 – 4.0
• Loss Tangent (tan δ) <= 0.005
Ten materials of various constructions from
multiple manufacturers were provided for char-
acterization. Table 1 presents these materials
and their general properties while assigning
each material an arbitrary designator.
The ɛ
r
of each was measured using eight dif-
ferent methods and where it is demonstrated in
this paper:
Microstrip Transmission Line Methods:
1. Extraction from impedance (ɛ
r
only)
2. Group delay extraction from phase
(ɛ
r
only)
3. Differential phase length (ɛ
r
only)
Free Space Transmission Method:
4. Free space quasi-optical (ɛ
r
only)
ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1)
Table 1: Circuit Board Materials Tested.